A process or task is a portion of a program
in some stage of execution. A program
can consist of several tasks.each process
that runs in an operating system is assigned
a process control block that holds
information about the process , such as a
unique ID(a number used to identify the
process),the saved state of the process,
the process priority and where it is located
in memory.
A process in a computer system may be in
one of a number of different possible
states, such as
ready - if it can run when the processor
becomes free
running - it currently has the processor
blocked - it cannot run when the processor
becomes free
When a running process is interrupted by
the processor after completing its allotted
time, its state is saved in its process control
block, its process state changed to ready
and its priority adjusted.
When a running process accesses an input
or output device, or for some reason
cannot continue, it is interrupted by the
processor, the process state and
associated data is saved in the associated
process control block. The process state is
changed to blocked and the priority
adjusted.
When the scheduler decides the next task to
run, it changes the process state of the
selected process to running and loads the
saved data associated with that process
back into the processor.
Typically, an operating system will provide a
number of program function calls that can be
used to control processes. These are similar to
those shown below,
block()
wakeup()
suspend()
sleep()
change_priority()
A process control block or PCB is a data
structure (a table) that holds information
about a process. Every process or program
that runs needs a PCB. When a user
requests to run a particular program, the
operating system constructs a process
control block for that program.
Typical information that is stored in a
process control block is
the location in memory of where the 
process is
the priority of the process 
a unique process identification number 
(called PID)
the current process state (ready, running, 
blocked)
associated data for the process 
Processes can intercommunicate by
sending message, data or code between
them. A process can use OS calls like
sendmessage() and getmessage() to
exchange messages.
Two processes might want to co-operate in
performing a particular task. For example a
process might want to print to document in
response to a user request, so it starts
another process to handle the printing and
sends a message to it to start printing.
Once the process handling the printing
request finishes, it sends a message back
to the original process, which reads the
message and uses this to pop up a dialog
box informing the user that the document
has been printed.
There are other ways in which processes
can communicate with each other, such
as using a shared memory space.
Sometimes a process may need to wait for
some other process to finish before it can
continue. In this instance, the two
processes need to be synchronized
together. There are a number of ways in
which this can be done. A common
method in operating systems is to use a
variable called a semaphore that only one
process can own at a time
There are two calls associated with a
semaphore, one to lock it and one to
unlock it. When a process attempts to lock
a semaphore, it will be successful if the
semaphore is free.
If the semaphore is already locked, the
process requesting the lock will be blocked
and remain blocked till the process that
has the semaphore unlocks it.
When that happens, the process that was
blocked will be unblocked and the
semaphore can then be locked by it.
.
A thread is a separate part of a process. A
process can consist of several threads,
each of which execute separately. For
example, one thread could handle screen
refresh and drawing, another thread
printing, another thread the mouse and
keyboard. This gives good response times
for complex programs. Windows NT is an
example of an operating system which
supports multi-threading.
Multi-tasking systems support foreground
and background processes (tasks). A
foreground task is one that the user
interacts directly with using the keyboard
and screen. A background task is one that
runs in the background (it does not have
access to the screen or keyboard).
Background tasks are usually used for
printing.